Researchers Zakhar A. Iakovlev, Akashdeep Kamra, and Mikhail M. Glazov from the Ioffe Institute in St. Petersburg, Russia, have recently published a study in Nature Nanotechnology that explores a novel mechanism for controlling exciton propagation in magnetic systems. Their work focuses on the magnon-exciton drag effect in a bilayer van der Waals antiferromagnetic semiconductor called CrSBr.
In their research, the team developed a microscopic theory to explain how magnons, which are quasiparticles associated with spin waves in magnetic materials, can interact with excitons, which are bound pairs of electrons and holes. The scientists found that magnons can tilt the layer magnetizations in CrSBr, enabling charge carrier tunneling that mixes intra- and interlayer excitons. This interaction modulates the exciton energy and gives rise to an effective exciton-magnon coupling.
The researchers derived the effective Hamiltonian of this coupling, based on their calculations of the magnon spectrum, which considered various interactions such as short-range exchange interaction between Cr-ion spins, single-ion anisotropy, and long-range dipole-dipole interactions. Notably, they discovered that the long-range dipole-dipole interactions produce a negative group velocity of magnons at small wavevectors.
Despite the relatively small renormalization of exciton’s energy and effective mass due to the exciton-magnon interaction, the team found that three key two-magnon processes—exciton-magnon scattering, two-magnon absorption by exciton, and two-magnon emission—are highly efficient. By solving the Boltzmann kinetic equation, they evaluated the short exciton-magnon scattering time, which is in the sub-picosecond range and decreases with the increase of magnon population. This suggests that exciton-magnon scattering is likely to dominate over other scattering processes related to exciton-phonon and exciton-disorder interactions.
The researchers demonstrated that magnons can efficiently drag excitons, resulting in a large and nearly isotropic exciton propagation that can significantly exceed the intrinsic anisotropic diffusion. Their results provide a theoretical basis for recent observations of anomalous exciton transport in CrSBr and establish magnon-exciton drag as a powerful mechanism for controlling exciton propagation in magnetic systems.
This research has practical implications for the energy sector, particularly in the development of advanced energy conversion and storage devices. By understanding and harnessing the magnon-exciton drag effect, scientists could potentially design more efficient solar cells, photodetectors, and other optoelectronic devices that rely on the control of exciton propagation. Additionally, this mechanism could be exploited in spintronic devices, which utilize the spin degree of freedom of electrons to perform computations and store information, offering potential improvements in energy efficiency and processing speed.
The study, titled “Boltzmann transport theory of magnon-exciton drag,” was published in Nature Nanotechnology.
This article is based on research available at arXiv.